Electron beam current regulator for a light valve

ABSTRACT

Electron beam current in a light valve is maintained at a uniform value by feedback apparatus which senses current at the medium upon which a raster is generated by the beam and controls the rate of electron emission from an electron gun in accordance with amplitude of sensed current. Initial cathode current surges, occurring upon each initiation of energization of the light valve, are suppressed within the feedback apparatus, leaving operation of the light valve essentially unaffected by such surges.

United States Patent 7/1968 Nielsen et al. 4/l969 l78/DlG. 29 l78/DlG. 29

[72] Inventors llllo-chol Lee; 3,392,236 Thomas P. L. Liu, both of Liverpool, N.Y. 3,437,749

Primary Examiner-Roy Lake Assistant Examiner-Palmer C. Demeo Attorneys-Marcia Snyder, W. .l. Shanley, Jr., Frank L.

m n. m 0 C .m t

0 UM 9 1 m s" -mm bMNG 0. 06 N mm N m P ma AFPA UQU 2247 [ill Neuhauser, Oscar B. Waddell and James B. Forman [54] ELECTRON BEAM CURRENT REGULATOR FOR A LIGHT VALVE 7 Claims, 1 Drawing Fig.

[52] US. 315/307, T C Electron beam current in a] ight valve is mainpparatus which senses pon which a raster is generated by the beam and controls the rate of electron emission from an elec- ,tained at a uniform value by feedback at current at the medium u tron gun in accordance with amplitude of sensed current. lnitial cathode current surges, oce

energization of the l 40x9 90 2 513 ir/m 3%U 3 n 0 5 .5 3 i "8 Q) 7 2 5 m 2 m m MR 8 m mm H m m3 wmme "2 m 0 H 3 m" a "r "a "e "S 1 m d Lfl hm. ll o 55 urring upon each initiation of ight valve, are suppressed within the feedback apparatus, leaving operation of the light valve essentially unaffected by such surges.

[56] References Cited UNITED STATES PATENTS 3/1949 Thalner........................

l78/DIG. 29

sim a ze. wzmmw .mw ha m 1 r BLANKING PU LSES ELECTRON BEAM CURRENT REGULATOR FOR A lLlGlI-l'll VALVE This invention relates to electron beam regulators, and more particularly to apparatus for maintaining electron beam current in a light valve at a substantially constant value.

A light valve suitable for optical projection of electronically generated images onto a remote display surface comprises, in a preferred embodiment, an evacuated enclosure containing an electron gun in predetermined alignment with a transparent disc. The disc is rotated through a reservoir of light modulating fluid to deposit a continuously replenished layer of fluid on the disc surface. An electron beam, generated by the electron gun, is scanned across a portion of the light modulating fluid layer by electrostatic beam deflecting and focusing means, so as to selectively deform the layer. The fluid deformations thus formed constitute optical diffraction gratings which, in conjunction with a Schlieren optical system, selectively control passage of light from alight source through the disc and through an output window in the enclosure envelope in order to create visible images at a remote display surface on which the light impinges.

The side of the rotating disc facing the electron source carries a thin film, comprising a transparent conductive coating, on which is supported the deformable fluid layer. The film is maintained at a constant potential which is positive with respect to potential of the electron beam in order to attract electrons to the fluid layer. The pattern of charge thus formed on the fluid layer by electrons impinging thereon causes the layer to deform in that pattern as a result of electrostatic attraction between electrons deposited by the electron beam and the relatively positive potential of the transparent conductive coating.

The aforementioned diffraction gratings are formed by directing the electron beam onto the fluid layer and horizontally deflecting the beam across the surface of the layer in successive, substantially parallel paths. By velocity modulating the beam with signals corresponding to two primary colors, typically red and blue, the speed of horizontal deflection along these paths is varied in a periodic manner at a frequency much greater than the frequency of occurrence of each scan line or parallel path, producing vertically directed diffraction gratings corresponding to the red and blue signals, respectively. In addition, horizontally directed diffraction gratings, corresponding to the green signal, are formed by the horizontal scan lines or parallel paths of the scanning electron beam. The horizontally directed diffraction gratings are wobble modulated; that is, the size of the spot formed by the beam is varied in accordance with green signal modulation.

Depth of fluid layer deformation in each diffraction grating is varied in accordance with density of charge deposited by the electron beam so as to produce corresponding variations in intensity of light passed by the respective gratings. That is, velocity modulation produces variations in density of charge deposited by the electron beam at uniformly spaced locations along each horizontal path by controlling time spent by the beam in scanning over each of these locations so as to control the total charge deposited at each location. Additionally, wobble modulation produces variations in concentration of charge deposited along each horizontal path by controlling the instantaneous density of electrons per unit of electron beam cross sectional area so as to control the total charge deposited along the longitudinal center of each increment of the horizontal path being scanned by the beam. Intensity of the electron beam accomplishing modulation of the aforementioned types is maintained at a constant value, in order to avoid variations in intensity of light emitted by the light valve resulting, for example, from spurious changes in electron emission rate of the electron source. A system of this type is described and claimed in W. E. Good et al. U.S. Pat. No. 3,325,592, issued June 13, 1967 and assigned to the instant assignee.

As the electron beam is scanned horizontally, vertical diffraction gratings are produced as a result of periodic negative charge deposition at the uniformly spaced locations along each horizontal scan path. Conveniently, both red and blue difi'raction gratings may be formed in this manner by velocity modulation resulting from superimposing a first carrier frequency, to produce red diffraction gratings, and a second carrier frequency, to produce blue diffraction gratings, onto the horizontal deflection voltage waveform which is typically of sawtooth configuration. In addition, a green difl'raction grating is formed by controlling the scanning beam with a third carrier frequency so that the natural grating formed by the horizontal deflection paths of the electron beam comprises the green grating. Wobble modulation of the green grating is accomplished by spreading out, or smearing, in a vertical direction, the scanning electron beam, so as to vary the concentration of charge in a line along the center of the scanning direction. For minimum modulation of green, representing the green dark field, the natural grating is virtually wiped out. Conversely, the full natural grating itself represents maximum green modulation. Any variation in intensity of the electron beam under these modulation conditions is detrimental to proper operation of the light valve in that undesired optical intensity variations are introduced into the displayed image.

In order to protect the light valve against damage due to surges in its cathode current, protection circuitry is normally employed to shut down the video projection apparatus whenever the cathode current exceeds a predetermined amplitude. Whenever the light valve is turned on, however, its cathode current momentarily exceeds this predetermined amplitude, tending to cause the protection circuitry to deenergize the apparatus. It is essential that such unnecessary shutdown by the protection circuitry be avoided, so as to allow operation of the light valve.

Accordingly, one object of the invention is to provide apparatus which permits display of images by a light valve while avoiding optical intensity variations in the displayed image due to intensity variations of an electron beam within the light valve.

Another object is to provide apparatus for maintaining an electron beam at constant intensity.

Another object is to provide apparatus for limiting surges in electron beam current each time the electron beam is turned Briefly, in accordance with a preferred embodiment of the invention, a system for maintaining a constant rate at which electrons, emitted by an electron gun containing a control grid therein, impinge on target means within a light valve so as to form optical diffraction gratings on the target means comprises circuit means producing an output signal of amplitude proportional to the rate at which electrons impinge on the target means, 'and reference means for establishing a signal of amplitude proportional to the desired rate of impingement of electrons on the target means. Comparator means are coupled jointly to the circuit means and the reference means so as to produce a grid control voltage in accordance with the difference in output signals of the circuit means and the reference means. Voltage limiter means are employed to couple the comparator means to the control grid of the electron gun so as to permit the grid to maintain the rate at which electrons impinge on the target means at a substantially constant level corresponding to the level selected by the reference means. The voltage limiter means prevents voltage on the control grid from exceeding a predetermined amplitude level.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawing in which:

The single FIGURE is a schematic: diagram of a system to regulate electron beam current in a light valve, constructed according to the invention.

DESCRIPTION OF TYPICAL EMBODIMENTS In the FIGURE, a light valve 10, including an electron gun I], is shown schematically. The electron gun includes an electron source, illustrated as a cathode 12, a control grid 13, and an anode 14 which contains an aperture 15 therein and is coupled to a source of positive high voltage. At the end of the light valve opposite electron gun portion 11 is situated a transparent disc 16 containing a thin, conductive film 17 formed thereon. A layer 18 of deformable fluid is coated on thin film 17, while the thin film is maintained at an averagepositive potential just slightly below the positive potential on anode 14 by a series circuit comprising a resistance 20 and a relaxation oscillator 21 energized by a source of positive high voltage of substantially the same amplitude as that applied to anode 14 of the light valve.

Relaxation oscillator 21 comprises a resistance 25, and a gas-filled glow discharge tube 22 connected in parallel with a series-connected resistance 23 and capacitance 24. Resistance 25 is connected in series with discharge tube 22 in order to provide a voltage drop of sufficient magnitude to prevent the high voltage supply from maintaining tube 22 in conduction indefinitely once it has fired. A capacitance 26 is connected across the series circuit comprising resistances 23 and 25 and capacitance 24 for the purpose of suppressing transients resulting from the firing and extinction of tube 22 by providing a short circuit to such transients, while resistance 20 serves to attenuate any transients which may not be completely suppressed within oscillator 21. As a result, the effect of transient potentials on thin film 17 of the light valve is minimized.

Output signals from relaxation oscillator 21, furnished from a junction 19 common to resistance 23 and capacitance 24, are AC coupled through a capacitance 27 to the input of a pulse frequencyto-amplitude converter 31. These signals are coupled through a current limiting resistance 32 to the gate of a field-efi'ect transistor 33, herein designated FET, which is employed as a buffer amplifier because its high input impedance does not unduly load the output of oscillator 21 and hence does not appreciably affect its frequency of operation. The FET is protected against any damagingly large voltage surges which pass through blocking capacitor 27 whenever the high voltage supply is turned on or off, by a pair of diodes 34 and 35 connected in reverse-biased, series-aiding relationship between a source of negative voltage 28 and a source of positive voltage 29. Voltage sources 28 and 29 are of substantially equal magnitude considerably below that of the high-voltage supply energizing oscillator 21. The junction common to diodes 34 and 35 is connected to the junction common to capacitance 27 and resistance 32. Thus, whenever the voltage at the junction common to capacitance 27 and resistance 32 tends to exceed the amplitude of positive voltage supply 29, diode 35 becomes forward biased and conducts in the forward direction, clamping the voltage at the junction of capacitance 27 and resistance 32 substantially to the amplitude of the positive voltage supply. Similarly, whenever the voltage at the junction common to capacitance 27 and resistance 32 tends to exceed the amplitude of negative voltage supply 28, diode 34 becomes forward biased and conducts in the forward direction, clamping the voltage at the junction of capacitance 27 and resistance 32 substantially to the amplitude of the negative voltage supply.

The input signal to F ET 33 is developed across a resistance 36 coupled between the gate electrode of FET 33 and ground. This signal is developed from negative-going pulses produced by oscillator 21. A zener diode 37, connected in parallel with resistance 36, limits the maximum negative potential of pulses applied to the gate of from 33 clipped oscillator 21 so that the pulses applied are all clipped to a uniform amplitude level. Positive drain bias is supplied to the drain electron D of PET 33 through a resistance 38, while a load resistance 40 is connected between the source electrode S of the PET and negative voltage supply 28.

Output signals from FET 33 are AC coupled through a capacitance 41 to the base of a PNP transistor 42. Base bias for transistor 42 is developed across a resistance 43 connected to ground, while the emitter of transistor 42 is connected directly to ground. Capacitance 50 is connected between the collector of transistor 42 and ground.

The collector of transistor 42 is connected to the collector of an NPN transistor 45 through a resistance 44. The emitter of transistor 45 is connected through a biasing resistance 46 to negative potential source 28. A zener diode 47 is connected in series with a resistance 48 between sources 29 and 28 of positive and negative potential, respectively, and base voltage for transistor 45 is furnished from the junction common to zener diode 47 and resistance 48. The zener diode maintains a substantially constant voltage on the base of transistor 45 and thus holds the voltage across resistance 46 fixed at a substantially constant value. As a result, collector current in transistor 45 remains substantially constant, so that the transistor functions as a constant current source.

The collector of transistor 42 is also connected to the base of a PNP transistor 53 through a transistor protection diode 51. Base bias is furnished to transistor 53 through a resistance 52, while collector bias is furnished to the transistor through a resistance 54 connected to negative voltage source 28. The emitter of transistor 53 is DC coupled to the base of an NPN transistor 57 in an amplitude comparator circuit 61 through a resistance 55. A capacitance 56 coupling the base of transistor 57 to ground acts to accumulate charge through transistor 53, rendering transistor 57 responsive to amplitude of voltage across capacitance 56.

Comparator 61 contains a second NPN transistor 58 having its emitter connected directly to the emitter of transistor 57, with emitter bias for both transistors being furnished through a resistance 60 from negative voltage source 28. Collector bias for each of transistors 57 and 58 is furnished through resistances 62 and 63, respectively, from positive voltage source 29. The base-to-emitter potential on each of transistor 57 and 58 is limited by a pair of diodes 64 and 65, respectively, each diode being connected in a reverse-biased direction between the emitter and base of each respective transistor. Base voltage of manually adjustable amplitude is furnished to transistor 58 from the centertap of a potentiometer 66 connected between negative voltage source 28 and ground. The collector of transistor 58 is coupled to ground through a capacitance 67 and resistance 68 in series, which function to dampen transients and provide a relatively smooth output signal from comparator 61 to an inverting amplifier 71.

The output of amplifier 71 is furnished to one side of a rheostat 72 connected in series with a zener diode 73. The anode of zener diode 73 is biased negatively by the light valve negative grid bias supply 75 through a coupling resistance 74. The magnitude of voltage furnished by negative grid bias supply 75 is greater than that of either of voltage sources 28 and 29. Rheostat 72, zener diode 73 and resistance 74 together comprise a voltage limiter 81, preventing cathode current in the light valve from exceeding a maximum value as determined by the setting of rheostat 72. Blanking pulses are coupled to grid 13 of light valve 10 through a coupling capacitor 82. An isolation resistance 76 is connected between the anode of zener diode 73 and grid 13 of light valve 10 in order to prevent the relatively low output impedance of voltage limiter 81 from un duly loading the source (not shown) of blanking pulses.

In operation, electrons emitted from cathode 12 of light valve 10 and accelerated by positive potential on anode 14 of the light valve are passed through aperture 15, which shapes them into a beam. Those electrons passing through aperture 15 are deposited on deformable medium 18, forming the desired deformations therein, having been attracted by the positive potential applied to thin film 17. The greater theiamplitude of negative charge on deformable medium 18, the greater the amplitude of electrostatic forces tending to deform medium 18, and vice versa. Amplitude of charge deposited on deformable medium 18, in turn, depends upon amplitude of negative voltage on grid 13 of light valve 10. That is, as negative voltage amplitude on grid 13 decreases, more electrons emitted by cathode 112 of the light valve reach deformable medium llii, so that the rate at which electrons impinge on the deformable medium increases, and vice versa. Electrons thus accumulated on deformable medium 118 eventually leak through the deformable medium to thin film 17. The total leakage current from thin film R7 is passed to relaxation oscillator 2ll. The purpose of the circuitry external to light valve MD, as shown in the FIGURE, is to maintain an electron beam of constant intensity within light valve 110 so that depths of vertically directed and horizontally directed defonnations in deformable medium 18 are responsive only to the previously mentioned velocity modulation and wobble modulation, respectively, of the electron beam.

Assuming that electron beam intensity is at the desired value, a predetermined current flows from relaxation oscillator 21 to thin film 17. This current determines the rate at which capacitance 24 acquires a charge; that is, the rate at which capacitance 24 charges is increased when current flow from relaxation oscillator 21 to thin film 117 increases, and vice versa.

When capacitance 24 has charged to a level where voltage across the capacitance has reached the firing potential of glow discharge tube 22, the tube fires, causing capacitance 24 to discharge through a conduction path comprising tube 22 and resistance 23;. The capacitive discharge continues until voltage across tube 22 falls to the extinction potential thereof. At this time, tube 22 ceases conduction, allowing capacitance 241 to again acquire a charge until the potential thereon reaches the firing potential of tube 22.

While capacitance 24$ is being charged, polarity of voltage across resistance 23 is such that junction W represents the positive side of resistance 23. However, when glow discharge tube 22 fires, the charge stored on capacitance 241 causes current flow through resistance 23 in a direction tending to drive the voltage at junction 11% abruptly in a negative direction. When glow discharge tube 22 reaches its extinction potential and terminates conduction, junction 19 abruptly swings in a positive direction. The net effect of these voltage changes is to produce a negative-going pulse at junction 19 in synchronism with the firing of glow discharge tube 22, with a pulse duration substantially equal to the conduction interval of tube 22. Capacitance 26, as previously mentioned, prevents transients due to fast switching of glow discharge tube 22 from affecting the high-voltage supply or the voltage on thin film 117.

Negative-going pulses thus produced by operation of relaxation oscillator 2ll are coupled through capacitance 27 and resistance 32 to the gate of field-effect transistor 33. These pulses are all of uniform amplitude, as determined by zener diode 37. During each interval between pulses relaxation oscillator 21, the base of transistor 42 remains substantially at ground potential. Transistor 432 is thus in a substantially nonconductive condition, permitting capacitance 50 to be charged with constant current from the collector of transistor US through resistance M. Thus, capacitance 56 is charged at a substantially linear rate with respect to time during each interval between successive pulses furnished from relaxation oscillator 2ll.

When a negative pulse is produced by relaxation oscillator 2H, conduction of lFlET 33 during the pulse interval is diminished so the potential on the source electrode of the FET swings in a negative direction during this time. This negative voltage swing is applied to the base of transistor 42 through capacitance M. As a result, base potential on transistor d2 becomes negative, so that the transistor becomes saturated and provides a low impedance path for abruptly discharging capacitance 50. Upon termination of the pulse from oscillator 211, capacitance 50 again begins to charge at a linear rate. The peak voltage across capacitance 50, which is reached at the end of the interval between any pair of pulses from oscillator 25, is thus directly proportional to elapsed time following termination of the pulse produced by the oscillator at the beginning ofthis interval.

During each interval between successive pulses from oscillater 21, the cathode of diode 51 is biased negatively due to voltage stored on capacitance 50, so that diode 51 is in a forward-biased condition. As a result, the increasing negative voltage on capacitance 50 causes the impedance of transistor 53 to decrease, allowing capacitance 56 to charge, through the collector-to-emitter circuit of transistor 53, up to a voltage of amplitude equal to the peak voltage reached on capacitance 50. Upon occurrence of a pulse from oscillator 21, the low impedance collector-to-emitter path presented by transistor 42 reverse-biases diode 51 and the base-to-emitter circuit of transistor 53. As a result, transistor 53 substantially ceases to conduct and the voltage stored on capacitance biases transistor 57 into conduction since the voltage on capacitance 56 is less negative than the voltage on the emitter of transistor 57. During conduction of transistor 57, capacitance 56 discharges very slightly through the relatively high impedance of the base-to-emitter circuit of transistor 57.

When the pulse from oscillator 21 is terminated,.

capacitance 50 once again begins to acquire a charge. in the event the amplitude of voltage on capacitance 50 exceeds that on capacitance S6, transistor 53 resumes conduction. Thus, any charge on capacitance 56 which may have leaked off through the relatively high impedance of the base-toH-emitter circuit of transistor 57 is replaced. The maximum amplitude of voltage across capacitance 50 is dependent upon the length of interval between pulses from relaxation oscillator 21, since the greater the interval the greater the time available for capacitance 50 to acquire a charge, and hence the greater the charge acquired. Conversely, the shorter the interval, the lesser the time available for capacitance 50 to acquire a charge, and hence the lesser the charge acquired. Due to the relatively long RC time constants involved in charging and discharging capacitance 56, however, the voltage thereon changes only at a very slow rate.

Transistor 58 is normally in a conductive condition since the base is maintained at a constant potential determined by the setting of potentiometer 66, which is less negative than the potential applied to the emitter of transistor '58 through resistance 60. As a result, collector current flow through resistance 63 causes a positive quiescent potential to appear at the collector of the transistor which, after being filtered by capacitance 67 and resistance 68, comprises the output signal of comparator 61. However, voltage on the collector of transistor 5% is affected by current flow through transistor 57 since both transistors are connected in common to emitter bias resistance 60. Thus, any increase in conduction of transistor 57 causes emitter voltage on transistor 58 to become less negative, thereby causing a decrease in conduction of transistor 58. Accompanying the decrease in conduction of transistor 58 is an increase in positive potential on the collector thereof. Conversely, any decrease in conduction of transistor 57 causes emitter voltage on transistor 58 to become more negative, thereby causing an increase in conduction of transistor 58. Accompanying the increase in conduction of transistor 58 is a decrease: in positive potential on the collector thereof. Thus, it is evident that an increase in frequency of pulses produced by oscillator 21 results in a more positive output voltage from comparator 61. Conversely, a decrease in frequency of pulses produced by oscillator 21 results in a less positive output voltage from comparator fill. In order to make changes in output signal from comparator 61 more gradual and hence more manageable, the collector voltage of transistor 58 is applied across the series circuit comprising capacitance 67 and resistance 68 so as to dampen any abrupt changes in collector potential. This avoids changes in electron beam intensity which might be sufficiently abrupt to cause the system to hunt.

Output voltage from comparator 61 is amplified by amplifier 7ll, and the resulting output current of amplifier 7i flows through resistance 74, providing a variable control grid voltage for light valve 110. The maximum control grid voltage swing is thus limited between the amplitude of negative grid bias supply 75 and the negative sum of the voltage across zener diode 73 which remains constant since the diode is biased in its reverse breakdown condition, and potentiometer '72. The series combination of zener diode 73 and potentiometer 72 limits the minimum negative grid voltage relative to the cathode and thus limits the maximum cathode current in the light valve.

Whenever light valve M) is switched on, no pulses are initially produced by relaxation oscillator 21. This absence of initial pulses causes output voltage of amplifier 711 to swing in a positive direction, allowing abnormally large voltage drops to appear across resistance 74!. The grid voltage of the light valve is consequently moved in a positive direction toward the cathode potential so as to require an increase in cathode current in order to maintain a stable closed-loop condition. This could result in cathode current overshoot and, due to the inherently long system time constant, damage to aperture of the light valve, in the form of a distortion of its shape, might occur within this transient period. The sum of the fixed voltage drop across zener diode 73 plus the voltage drop across potentiometer 72, however, limits the maximum amplitude of current overshoot to a safe value, preventing such damage from occurring. Moreover, if the regulator system itself should fail for any reason, light valve cathode current would be limited to the safe valve; alternatively, potentiometer 72 could be manually readjusted to obtain the optimum electron beam current.

The foregoing describes apparatus which permits display or images by a light valve without optical intensity variations in the displayed image due to intensity variations of an electron beam at constant intensity and limits surges in electron beam current each time the electron beam is turned on.

While only certain preferred features of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

We claim: 1. A system for maintaining a constant rate at which electrons, emitted by an electron gun containing a control grid therein, impinge on target means within a light valve, said electrons forming optical difi'raction gratings on said target means so as to allow optical projection onto a display surface of image representations formed by modulation of said gratings, said system comprising:

circuit means coupled to said target means for producing an output signal of amplitude proportional to the rate at which said electrons impinge on said target means;

reference means for establishing a signal of amplitude proportional to the desired rate of impingement of electrons of said target means;

comparator means responsive jointly to said output signal of said circuit means and said desired signal of said reference means for producing a grid control voltage; and voltage limiter means coupling said comparator means to said control grid so as to permit said grid to maintain the rate at which electrons impinge on said target means at a level determined by said reference means, said voltage limiter means preventing voltage on said control grid from exceeding a predetermined amplitude level.

2. The system of claim 1 wherein said voltage limiter means includes a source of negative grid bias, resistance means coupling said source of negative grid bias to said control grid, and zener diode means coupling said comparator means to said control grid.

3. The system of claim 1 wherein said circuit means comprises pulse-generating means coupled to said target means, said pulse-generating means producing pulses at a frequency rate determined by the rate at which said electrons impinge on said target means, and pulse frequency-to-amplitude conversion means coupled to said pulse-generating means for producing said output signal of amplitude proportional to the frequency rate of said pulses.

4. The system of claim 3 wherein said voltage limiter means includes a source of negative grid bias, resistance means coupling said source of negative grid bias to said control grid, and zener diode means coupling said comparator means to said control grid.

5. The system of claim 3 wherein said pulse-generating means comprises a series circuit coupled to said target means, said series circuit including resistance means and capacitance means, and means connected in parallel with said series circuit so as to form a low-impedance circuit path in shunt with said resistance means and said capacitance means when voltage across said capacitance means rises to a first predetermined value and to substantially terminate said low-impedance circuit path when voltage across said capacitance means falls to a second predetermined value of lower amplitude than said first predetermined value.

6. The system of claim 3 wherein said pulse frequency-toamplitude conversion means includes first capacitance means, means coupled to said first capacitance means for providing substantially constant amplitude current to said first capacitance means during the entire interval between each two successive pulses produced by said pulse-generating means, means responsive to said pulse-generating means for discharging said first capacitance means during the entire duration of each pulse produced by said pulseH-generating means, second capacitance means, and charging means coupled to said second capacitance means, said charging means being responsive to voltage on said first capacitance means so as to regulate the voltage on said second capacitance means in accordance with the frequency of pulses produced by said pulse-generating means.

7. The system of claim 6 wherein said pulse-generating means comprises a series circuit coupled to said target means, said series circuit including resistance means and third capacitance means, and means connected in parallel with said series circuit so as to form a low-impedance circuit path in shunt with said resistance means and said third capacitance means when voltage across said third capacitance means rises to a first predetermined value and to substantially terminate said low-impedance circuit path when voltage across said third capacitance means falls to a second predetermined value of lower amplitude than said first predetermined value. 

1. A system for maintaining a constant rate at which electrons, emitted by an electron gun containing a control grid therein, impinge on target means within a light valve, said electrons forming optical diffraction gratings on said target means so as to allow optical projection onto a display surface of image representations formed by modulation of said gratings, said system comprising: circuit means coupled to said target means for producing an output signal of amplitude proportional to the rate at which said electrons impinge on said target means; reference means for establishing a signal of amplitude proportional to the desired rate of impingement of electrons of said target means; comparator means responsive jointly to said output signal of said circuit means and said desired signal of said reference means for producing a grid control voltage; and voltage limiter means coupling said comparator means to said control grid so as to permit said grid to maintain the rate at which electrons impinge on said target means at a level determined by said reference means, said voltage limiter means preventing voltage on said control grid from exceeding a predetermined amplitude level.
 2. The system of claim 1 wherein said voltage limiter means includes a source of negative grid bias, resistance means coupling said source of negative grid bias to said control grid, and zener diode means coupling said comparator means to said control grid.
 3. The system of claim 1 wherein said circuit means comprises pulse-generating means coupled to said target means, said pulse-generating means producing pulses at a frequency rate determined by the rate at which said electrons impinge on said target means, and pulse frequency-to-amplitude conversion means coupled to said pulse-generating means for producing said output signal of amplitude proportional to the frequency rate of said pulses.
 4. The system of claim 3 wherein said voltage limiter means includes a source of negative grid bias, resistance means coupling said source of negative grid bias to said control grid, and zener diode means coupling said comparator means to said control grid.
 5. The system of claim 3 wherein said pulse-generating means comprises a series circuit coupled to said target means, said series circuit including resistance means and capacitance means, and means connected in parallel with said series circuit so as to form a low-impedance circuit path in shunt with said resistance means and said capacitance means when voltage across said capacitance means rises to a first predetermined value and to substantially terminate said low-impedance circuit path when voltage across said capacitance means falls to a second predetermined value of lower amplitude than said first predetermined value.
 6. The system of claim 3 wherein said pulse frequency-to-amplitude conversion means includes first capacitance means, means coupled to said first capacitance means for providing substantially Constant amplitude current to said first capacitance means during the entire interval between each two successive pulses produced by said pulse-generating means, means responsive to said pulse-generating means for discharging said first capacitance means during the entire duration of each pulse produced by said pulse-generating means, second capacitance means, and charging means coupled to said second capacitance means, said charging means being responsive to voltage on said first capacitance means so as to regulate the voltage on said second capacitance means in accordance with the frequency of pulses produced by said pulse-generating means.
 7. The system of claim 6 wherein said pulse-generating means comprises a series circuit coupled to said target means, said series circuit including resistance means and third capacitance means, and means connected in parallel with said series circuit so as to form a low-impedance circuit path in shunt with said resistance means and said third capacitance means when voltage across said third capacitance means rises to a first predetermined value and to substantially terminate said low-impedance circuit path when voltage across said third capacitance means falls to a second predetermined value of lower amplitude than said first predetermined value. 